Optically positive cordierite from the khondalites of Pachipenta, Vizagapatam District

Summary and conclusion

An optically positive cordierite from the khondalites in Pachipenta of Vizagapatam District was chemically analysed and its optical properties determined. It has a hardness of 7·5 and Sp. Gr. 2·71 and its 2V as determined by Fedorov’s Stage is 74°. In contrast to the negative optical sign of a normal cordierite, this cordierite gives a positive optical sign. The tentative suggestion put forward by Krishnan and others that the optically positive character of cordierites is probably due to the ratio of MgO to FeO does not find support from the present studies. As shown by Winchell it appears that there is no close correspondence between chemical composition and optical characters in cordierites. The possibilities of traces of alkalies contributing to the change in optical sign as suggested by Folinsbee, is worth consideration but must await the availability of analyses giving these constituents.